The present invention relates to a variable gain circuit, more particularly, to a variable gain circuit which is configured such that the gain exponentially varies with regard to the gain control signal, a low electric power is consumed, and it can be applied to portable wireless devices.
This application is based on Japanese Patent Application No. 10-370290, filed Dec. 25, 1998, the entire content of which is incorporated herein by reference.
In recent years, developments of wireless communication apparatuses represented by portable or mobile telephone units have been made vigorously. These wireless communication apparatuses are required to be small sized and lightweight, as they are used by carrying by man or in the form of being loaded on automobile. Accordingly, as the parts which constitute the apparatuses there are strongly desired the monosilic IC (integrated circuit) parts which are suited to make small sized, lightweight, and low power consuming type, rather than the conventional parts of hybrid constitution formed by connecting many units of constituting parts. Besides the miniaturization of parts, price reduction of apparatuses is naturally required, for which formation of monosilic IC type is a technique essential for price reduction.
In a wireless transmitting/receiving circuit in such wireless communication apparatus, a variable gain amplifier is included in an IF (intermediate frequency) stage, and the IF signal can be adjusted to a moderate level by the variable gain amplifier. According to a wireless communication apparatus of CDMA (code divisional multiplex access), because the transmission gain circuit of this IF stage is required to carry out an extensive gain control that permits a signal level control for 70 dB or more.
In general, in order to carry out a gain control of such extensive range, it is required to make adjustment of the signal level in exponential function to the gain control signal. However, as described below, in a conventional variable gain amplifier, the range in which the signal level can be adjusted in exponential function to the gain control signal is considerably limited and it is difficult to respond to the above requirement, and there is a problem that the control becomes difficult when it is intended to change the gain in excess of this range.
A Gilbert type variable gain amplifier is shown in FIG. 1. Transistors Q100 and Q101 constitute a differential transistor pair, wherein an input signal current Isig is inputted to the common emitter terminal, and the output signal current Ia is taken out from the collector terminal of one transistor (herein, Q100). In order to form an output signal current Ia by multiplexing the input signal current I.sub.sig by a predetermined gain multiple, a gain control signal Vx is inputted between the base terminals of the transistors Q100 and Q101. The current (I.sub.sig -Ia) flowing to the collector terminal of the other transistor (herein, Q101) is regarded as unnecessary current, and is designed to flow into th e power source Vcc or the like.
The gain of this variable gain amplifier, i.e., a transfer function from the input signal current I.sub.sig to the output signal current Ia, is approximately represented by the following Equation 1: EQU Ia/I.sub.sig =1{1+exp (Vx/Vt)} (1)
where, Vt is a thermal voltage, which is approximately 26 mV at a room temperature. PA1 where, Vt is a thermal voltage, and b.gtoreq.0 PA1 a gain control circuit which controls the gain of the input signal according to the corrected gain control signal Vy, wherein the gain control circuit has a transfer function to be represented by the following equation: EQU Is/I.sub.sig =1/{1+exp (Vy/Vt)} PA1 wherein the input signal current is I.sub.sig, and the output signal current is Ia. PA1 a gain control signal correction circuit which corrects in the following manner a gain control signal Vx which varies linearly to the gain in decibel equation to obtain the corrected gain control signal Vy: EQU Vy=Vt.times.ln{exp (b.times.Vx/Vt)-1} PA1 where, Vt is a thermal voltage, and b.gtoreq.0 PA1 a gain control circuit which controls the gain of the input signal according to the corrected gain control signal Vy, wherein the gain control circuit comprises a differential transistor pair in which an input signal current is inputted to the common emitter terminal, a corrected gain control signal is supplied to a point between the base terminals of the two transistors, and an output signal current is taken out from the collector of one transistor. PA1 a gain control signal correction circuit which corrects in the following manner a gain control signal Vx which varies linearly to the gain in decibel expression to obtain the corrected gain control signal Vy: EQU Vy=Vt.times.ln{exp (b.times.Vx/Vt)-1} PA1 where, Vt is a thermal voltage, and b.gtoreq.0 PA1 a gain control circuit which controls the gain of the input signal according to the corrected gain control signal Vy, wherein the gain control circuit comprises a first differential transistor pair in which a positive input signal current is inputted to the common emitter terminal; a corrected gain control signal is supplied to a point between the base terminals of the two transistors; and an output signal current is taken out from the collector of one transistor, and a second differential transistor pair in which a negative input signal current is inputted to the common emitter terminal; a corrected gain control signal is supplied to a point between the base terminals of the two transistors; and an output signal current is taken out from the collector of one transistor.
Under the condition of 1&lt;&lt;exp (Vx/Vt), Equation 1 can be approximated to Ia/I.sub.sig =1/exp (Vx/Vt), so that it is known that the gain varies (decreases) in exponential function to the gain control signal Vx. However, in case the conditions of 1&lt;&lt;exp (Vx/Vt) do not hold good, for example, in the region where the gain control signal Vx is no more than zero, the relation between Vx and gain becomes, as shown in Equation 1: 1/{1+exp (Vx/Vt)}, which is not the relation to change in exponential function. Namely, when the assumption of 1&lt;&lt;exp (Vx/Vt) does not hold good to the gain control signal Vx, then the change of the gain to the gain control signal Vx becomes no longer exponential function like. The state of change in the gain G to the gain control signal Vx is shown in FIG. 2. In this manner, down to a certain level of the gain control signal Vx the gain G (dB) increases linearly in proportion to the decrease of the control signal Vx, but under the certain level, the rate of decrease in gain G (dB) is lowered in comparison with the decrease in the control signal Vx.
In case of carrying out a gain control in the wireless communication apparatus or the like, there is required from the point of facility of control that the gain is changed in exponential function to the gain control signal Vx, in other words, that the relation between the gain control signal Vx and the gain in decibel expression is linear type. However, as described above, in the variable gain amplifier of FIG. 1, such a linear-in-dB relation is obtainable only in the range for Vx to satisfy the conditions of 1&lt;&lt;exp (Vx/Vt), so that the gain control cannot be made by extensively changing Vx. Furthermore, the gain Ia/I.sub.sig with which such linear-in-dB relation can be obtained is no more than 1/2 in the maximum, which means to discard about half the IF signal which is an input signal current I.sub.sig, so that there is a problem of lowering S/N ratio of the signal outputted from the variable gain circuit.
As described above, the conventional variable gain amplifier has had a narrow range in which the gain can be controlled in exponential function to the gain control signal, thus involving a problem that the control becomes difficult when gain control is attempted in excess of the range.